<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    * {
      margin: 0;
      padding: 0;
    }

    /* .box {
      border-top: 100px solid lightblue;
      border-right: 100px solid lightcoral;
      border-bottom: 100px solid lightpink;
      border-left: 100px solid lightgray;
    } */
    .box {
      width: 0;
      height: 0;
      border-top: 100px solid lightblue;
      border-right: 100px solid transparent;
      border-bottom: 100px solid transparent;
      border-left: 100px solid transparent;
    }
  </style>
</head>

<body>
  <div class="box"></div>
</body>

</html>